Turbochargers are used in numerous applications such as automotive, marine, and aerospace applications. Turbochargers may provide a pressure increase (or boost) in the intake air over atmospheric pressures to increase the power output of the engine. A turbocharger may include a turbine section having a turbine wheel, a compressor section having a compressor wheel, and a bearing section that houses a shaft interconnecting the turbine wheel and the compressor wheel. The turbine wheel may extract energy from exhaust gases to drive the compressor via the interconnecting shaft, while the compressor wheel may increase the pressure of intake air for delivery to the combustion chamber of the engine.
Some turbocharger designs may include a wastegate that opens to divert exhaust gases away from the turbine wheel to the exhaust system when the turbocharger boost reaches a pressure threshold. In contrast, variable turbine geometry (VTG) turbochargers may not require a wastegate. Instead, in VTG turbochargers, the turbine output may be regulated with guide vanes that surround the turbine wheel at the turbine inlet. The guide vanes may open and close to control the flow of exhaust gases onto the turbine wheel in order to maximize turbocharger boost under varying operating conditions.
A pneumatic or electronic actuator may control the opening and closing of the guide vanes via an actuation pivot shaft (APS) housed in the bearing section. The APS may be received in a bushing held in a bore of the bearing section, with a head end of the APS penetrating into the turbine section for exposure to high temperature and high pressure exhaust gases. A pre-defined clearance between the bushing and the APS may exist to allow tilting and rotation of the APS in the bushing, and to prevent binding of the APS in the bushing caused by thermal growth and/or corrosion build-up. However, the clearance between the APS and the bushing may provide a flow path for fugitive exhaust gases that have not been treated by the exhaust aftertreatment system to escape from the turbine section into the atmosphere.
Current VTG turbochargers may include piston rings that partially obstruct the leakage of exhaust gases through clearances in the APS/bushing assembly. Development of more effective seals that completely eliminate exhaust gas leakage is challenging, however, due to the high temperature and corrosive environment of the APS/bushing assembly which requires sealing materials with high temperature capabilities and resistance to corrosion. In addition, suitable seals should have a service life that matches the service life of the vehicle, and should be flexible enough to permit tilting and rotation of the APS in the bushing without compromising the effectiveness of the seal. The latter requirement precludes the use of some rigid face seals that may become unseated as the shaft tilts relative to the bushing.
U.S. Pat. No. 7,644,583 discloses a variable nozzle turbocharger (VNT) having a control rod received in a bushing that is involved in actuating rotation of the turbine vanes. The control rod/bushing assembly includes washers on one end of the bushing, and a spring on the other end of the bushing to seal exhaust gas leakage therethrough. While effective, there is still a need for improved seal designs that seal leakage of exhaust gases through the APS/bushing interface in VTG turbochargers.